A. Field of the Invention
The present invention relates to a power transmission device, and in particular to a power transmission device having a variable speed changing device including a continuous speed changing unit.
B. Description of the Background Art
Power transmission devices for automobiles may be categorized into a variety of groups. One group of transmission devices includes the combination of a user activated mechanical clutch and a gear train commonly referred to as a standard or manual transmission (MT). Another group of transmission devices includes the combination of an automatic transmission device having a torque converter and a hydraulically controlled gear train. Such an automatic transmission device will be referred to herein after as an "AT" device. Another group of transmission devices includes the combination of an automatic clutch device, which can be one of various kinds of clutches, and a variable speed changing device, such as a continuous speed changing belt-pulley configuration. Such a variable speed changing device will be referred to hereinafter as a "CVT" device (Continuously Variable Transmission).
A CVT device generally is configured to allow for a continuously variable speed ratio, where the ratio of an engine's output rotation speed to the output of the transmission device is continuously variable. Such CVT devices usually include belts which have a large torque transmission capacity.
In a power transmission device provided with the CVT device, an engine is coupled to the CVT device by an automatic clutch device such as an electromagnetic clutch. However, the automatic clutch device such as an electromagnetic clutch has disadvantages, and for example, it will not allow a vehicle to creep forward at an extremely slow speed. The sudden engagement of electromagnetic clutches precludes the possibility of a vehicle creeping forward. In order to improve driving characteristics of the vehicle, a CVT device is combined with a torque converter in some cases.
CVT devices are often used in small cars where space is at a premium. Therefore, it is important to minimize the space taken by all components of the drive train. A problem with torque converters used with CVT devices is that the torque converter is large and takes up a fair amount of space.
Further, when the torque converter is combined with the CVT device, the capacity of the torque converter is often considered and determined in a manner similar to torque converters used with an AT device. In determining the capacity of a torque converter, a required capacity coefficient C is calculated based upon torque characteristics of the engine so that performance of the engine may be maximized. The capacity coefficient C represents the toque which can be inputted into the torque converter at a certain rotational speed, and is expressed as follows: EQU C=Ti/Ni.sup.2
where Ti represents an input torque, and Ni represents an input rotation speed. The capacity coefficient C is proportional to the outer diameter of a torus, the torus being defined by, for instance, the turbine of the torque converter. The capacity coefficient C is proportional to the outer diameter D of the torus to the fifth power. The capacity coefficient C may also be expressed as follows: EQU C=Ti/Ni.sup.2 =K.multidot.D.sup.5
where K is a coefficient depending on a type and a structure of the torque converter as well as properties of fluid and the speed ratio of input to output of the torque converter. Based on the above relationships, the size of the torque converter is generally determined from an estimated value of the outer diameter of the torus (the outer diameter of the turbine of the torque converter).
A CVT device is typically smaller than an AT device having a similar capacity. Even so, it is desirable to reduce the size of the torque converter used with the CVT device.